Tears have an integral role in the functioning of the eye. They protect the eye from infection, lubricate the movements of the eye and eyelid, and also ensure the presence of a thin transparent layer on the surface of the conjunctiva. The tear film’ is crucial for the functioning of the eye as an optical organ and the emotional role of tears can also not be overlooked. The mechanism by which tears are formed, distributed and drained will be discussed in the article below. The anatomy and function of the components will also be highlighted, as well as their anatomical relationships. We will conclude with review questions to test the reader’s understanding of the article content.
The function of the lacrimal apparatus is to produce tears, channel them to the surface of the eye, where they maintain moisture and flush debris and waste material from the ocular surface. The control of tears is under a multitude of inputs and sensory responses. When the ophthalmic branch of the trigeminal nerve (which supplies sensation to the eye) detects something on the eye e.g. dirt, or debris, we have an involuntary response of shutting our eyes (via the seventh cranial nerve i.e. the facial nerve), by the orbicularis oculi contracting. There are three types of tears. Basal tears are normally present and maintain lubrication and a general functioning of the eye. There are reflex tears that are generated due to irritation of the eye from dirt of debris. The third type is psychic tears/crying, which are formed under emotional control. Circulating hormones also influence the production of tears.
Conjunctiva- This is a thin semi-transparent mucus membrane. It covers the sclera (the white part of the eyeball), and is composed of non-keratinized stratified squamous and columnar epithelium. There are blood vessels within the conjunctiva that deliver blood to the ocular surface, and provide nutritional support. The mucins are secreted from specialized goblet cells on the surface of the conjunctiva. There are three parts to the conjunctiva. The palpebral part lines the internal surface of the eyelids themselves. The bulbar section covers the eyeball. The fornix section is the flexible region that connects the tarsal and bulbar parts, and allows the eye to move with freedom.
Lacrimal gland- This gland is about the size of an almond, and sits within the lacrimal fossa, located in the superior and outer edge of the orbital roof. The gland is divided into two sections anatomically. These are the small palpebral portion that lies closer to the eye, and the orbital portion that forms around four ducts. These ducts then combine with the 6 ducts of the palpebral portion, and are secreted onto the surface of the eye. The lacrimal gland is composed of cells that produce proteins and electrolytes, and cause water to follow by osmosis.
The blood supply of the gland is from the lacrimal artery (a branch of the ophthalmic artery), with the venous drainage through the superior ophthalmic vein. The gland is innervated by the parasympathetic lacrimatory nucleus of the facial nerve (which originates in the pons). Which leaves the skull via the foramen lacerum by hitching a ride with the greater petrosal nerve, eventually reaching the eye by merging with the lacrimal and zygomatic divisions of the ophthalmic division of the trigeminal nerve. The nerve also synapses in the pterygopalatine ganglion. The sympathetic innervation arises from the superior cervical ganglion, before merging to form the deep petrosal nerve, which merges with the greater petrosal nerve.
Lacrimal canaliculi/canals- These are small channels that lie in each eyelid, and commence at the puncta lacrimalia; small openings where the tears are drained from the surface of the eye. These canaliculi are divided into the superior duct and the inferior duct that drain into the lacrimal sac. They are lined with stratified squamous epithelium.
Lacrimal sac- This is the upper dilated end of the nasolacrimal duct. It connects to the lacrimal canaliculi which function to drain the tears from the eyes surface to the nasal cavity via the nasolacrimal duct. The cells that line these canaliculi are stratified columnar epithelium, with goblet cells.
Nasolacrimal duct- These drains the tears into the nose, and are drained away just anteroinferiorly to the inferior nasal concha. The cells that line this duct are stratified columnar epithelium. The membrane at the end of the tear duct (the valve of Hasner) may fail to open at birth, resulting in obstruction of the duct.
Lipids (meibum) are secreted by these specialized sebaceous glands and they form a part of the tear film. There are approximately 50 glands on the upper eyelid, and 25 on the lower lid. These are squeezed from the glands upon blinking, and they have numerous functions including closing the eyelid airtight and also prevent tear spillage onto the cheek, by maintaining the tears between the oiled edge of the eyelid and the eyeball.
The tear film consists of lipids, water and mucins. Mucins are both lipids and long hydrophilic molecules and are made by goblet cells that are scattered over the surface of the conjunctiva. The tear film is able to maintain a wet layer as it binds to the corneal and conjunctival epithelium, which is coated with a layer of mucins known as the glycocalyx. Tears wet both the front of the eye, and the inner surface of the upper eyelid. The relatively thin tear layer on the surface of the eye is covered by an oily layer that prevents tears from evaporating. This oily layer of lipids is largely composed of nonpolar lipids secreted by tarsal glands. The function of the lipid layer is dependent on the inner layer of lipids that are composed of both hydrophobic and hydrophilic lipids. As a result the layer can form an interface with the underlying aqueous fluid. The aqueous layer contains igA antibodies, electrolytes and antibacterial enzymes that protect the eye from infection. The tear film functions to protect the eye from shear forces during eye movements and blinking, maintains a smooth transparent optical layer and also shield the eye surface from environmental insults. The negative charge of the tear film also functions to repel negatively charged bacteria. When we feel sad, there is a large increase in lacrimal output, which dilutes the tear film. This then results in more watery tears that therefore flow more readily.